To function effectively, many systems require an accurate knowledge of the rotation speed difference between the rotation speeds of two rotating elements (wheels, disks, etc.).
Accordingly, in a system for driving an aircraft wheel in which the wheel is driven in rotation by an electric drive motor, the speed of the motor is slaved to the rotation speed of the wheel before coupling an output shaft of the motor to the wheel. It is therefore necessary to know the speed difference between the rotation speed of the motor and the rotation speed of the wheel.
Each of the two rotating elements is generally provided with an absolute speed sensor that measures the speed of each rotating element relative to the same fixed frame of reference, after which the difference between the speed of each rotating element is calculated.
There is obtained in this way a rotation speed difference measurement subject to the inaccuracies added by each absolute speed sensor and dependent on the drift of each absolute speed sensor.
A similar problem arises in measuring the relative angular position between two rotating elements.